An extension to HTML was created called XHTML. The fundamental idea of XHTML is that data can be declared context sensitive. Data that is context sensitive is only presented to the user when the right context is given.
A HTML document can be seen as a sequence of data. Every single item of data may be assigned its own context, the "data context". Each data items context is indepedent from the contexts of all other items. The context that defines which data items are shown is given by the user for each document that is requested through the selection of a link. This context will be called the current context.
A context is defined by a number of context attributes. A context attribute consists of a name and a set of values. These names and values do not have to be predefined, they can be freely chosen by the creator of the document. Only the value "default" is reserved.
Every time a context sensitive document is accessed the current context is checked against each individual data context. Whenever the match condition succeeds (see next chapter) the data item is presented.